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ditorials89 0021-7557/10/86-02/89
Jornal de Pediatria
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T
he sweat test still is the pivotal test for the diagnosisof cystic ibrosis (CF), even in the era of genetic testing based on genome DNA analysis. The classic technique by Gibson & Cooke,1 which still is considered the gold standard for sweat testing, has been repeatedly criticized because of its complexity, which restricts access: the complications of this technique include the need to elute the sweat from ilter paper or gauze used to collect
it, weighing samples twice before and after collection and subsequent dilution for chemical analysis designed to detect the two electrolytes whose concentration is abnormally high in CF, namely chloride and sodium. For this reason, attempts have been made to set up a simpler test to measure electrolyte
concentration in sweat. Qualitative and semiquantitative tests, such as agar plate or paper imprint testing, as well as those that measured chloride concentration directly on the skin,2 are history, as they were rapidly discarded. Two systems based on the indirect measurement of electrolytes in undiluted sweat collected in plastic capillary tubes (Macroduct®) after stimulation have aroused considerable interest: the osmometry and conductivity systems. The irst one measures total osmolarity, which expresses the total concentration of solutes in sweat, both electrolytes and non electrolytes: up to now the method has not been very successful and a few preliminary studies3 have not been continued. It has been discarded by the guidelines of the CF Foundation.2 The conductivity system has been more widely used. In a nutshell, it measures the capacity of sweat to conduct electrical current (conductivity), in milliamperes with a microamperimeter, which depends on
electrolyte concentration: as the dominant electrolytes in sweat are Cl- and Na+, this method transforms the current measured in NaCl equivalents, arbitrarily assuming that all ion concentration in sweat is due to Cl- and Na+ and simply dividing by two the total number of milliequivalents (or millimoles) calculated by measuring electrical current.4 In this way, the derived concentration value is slightly
higher than the value obtained by direct chemical analysis of chloride or sodium ions, because electrolytes other than Cl- and Na+ are inevitably included. However, a certain number of studies have been conducted that demonstrate that there is a close correlation and fair concordance (especially for the lowest values) between NaCl equivalent values measured by conductivity analysis and those obtained by quantitative chemical analysis of Cl- or Na+.5-8
The conductivity approach on sweat collected after stimulation by pilocarpine iontophoresis in a spiral capillary tube is claimed to be easier than the classic test by Gibson and Cooke, as it avoids several steps and risks of making mistakes or of failure involved in the classic method, which requires experienced staff and analytical procedures that can be carried out only in adequately equipped laboratories run by competent staff. Reassuring data have been provided regarding precision, accuracy, and reproducibility: regarding reproducibility, repeated tests on the same subject give similar results.7 Reassuring data have been provided also regarding its sensitivity (ability to identify a patient with CF as ill) and its speciicity (its ability to identify a healthy subject as healthy).5-9 However, here there is some uncertainty regarding the criteria adopted in the studies that addressed
* MD. Professor, Pediatrics. Scientific Director, Italian Cystic Fibrosis Research Foundation. No conflicts of interest declared concerning the publication of this editorial.
Suggested citation: Mastella G. Sweat testing: can the conductivity analysis take the place of the classic Gibson and Cooke technique? J Pediatr (Rio J). 2010;86(2):89-91.
doi:10.2223/JPED.1997
sweat testing: can the conductivity analysis
take the place of the classic Gibson and Cooke technique?
90 Jornal de Pediatria - Vol. 86, No. 2, 2010
references
1. Gibson LE, Cooke RE. A test for concentration of electrolytes in sweat in cystic ibrosis of the pancreas utilizing pilocarpine iontophoresis.Pediatrics. 1959;24:545-9.
2. Le Grys VA, Yankaskas JR, Quittell LM, Marshall BC, Mogayzel PJ Jr; Cystic Fibrosis Foundation. Diagnostic sweat testing: the Cystic Fibrosis Foundation Guidelines. J Pediatr. 2007;151:85-9. 3. Schoni M, Kraemer R, Bähler P, Rossi E. Early diagnosis of
cystic ibrosis by means of swear microsmometry. J Pediatr. 1984;104:691-4.
Sweat testing - Mastella G
the issues. In the study that is published in this issue,10 for instance, the calculation of sensitivity and speciicity of the conductivity test is based on thresholds found in the literature, not derived from experimental data obtained in the study: for the classic test Cl- < or > 60 mmol/L; for the conductivity test NaCl equivalents < or > 90 mmol/L. The fact that there is a not negligible number of CF subjects with borderline values of Cl- or Na+ measured by the classic test is ignored and such subjects were lacking in the study sample. Therefore, if the classic method is considered the gold standard with which the results of an alternative test are to be compared, sensitivity and speciicity should be assessed not in terms of two categories (CF diagnosis yes/ no), as in the study mentioned above, but rather in terms of three categories: normal (< 40 or < 30 mmol/L in infants), pathological (> 60 mmol/L, > 50 in infants), borderline (40-60 mmol/L over 6 months, 30-50 mmol/L during the irst 6 months of life). A few discrepancies between the two techniques have been found with borderline results.7
Worthy of note is also the success index in the collection of sweat with the Macroduct® capillary tubes, in view of the fact that the conductivity test is not reliable on sample collections smaller than 15 microlitres5-7 and that the classic test requires at least 50 microliters (with a collection area covering 9-10 cm2,but at least 75 microliters with a collection area covering 12-13 cm2).7,11 In the study by Mattar et al.,10 the minimum quantity of sweat required for the conductivity analysis was not speciied and the collection failure rate amounted only to 3-5 vs. 24-36% with the classic system. However, the CF subjects examined were older than 18 months (the age of the subjects without CF was not provided) and the collection time after stimulation lasted 30 to 60 minutes. We know that protracted collection time may favor the success rate, but may also alter electrolyte concentration: for this reason the recommended standard time is 30’.2,11,12 Other studies suggest that the failure rate with Macroduct® collection is higher, especially in the irst months of life, and that the success rate is higher with the classic system.6,7 An attempt to increase the success rate was made with the Nanoduct® method, which stimulates, collects, and analyzes sweat in a single step, while the electrodes and conductivity sensors are attached to the patient13,14: the procedure is claimed to last 15 minutes and to require at least 3 microliters of sweat. Also this test had acceptable diagnostic accuracy, although the irst attempts were disappointing on account of the excessively high false negative rate and the low success rate.13 However, also this method has notable feasibility limitations in neonates and infants, and we still do not have systematic data in non specialized clinical settings.
The validity of the Macroduct® collection system instead of ilter paper or gauze collection has been acknowledged by the CF Foundation for some time. However, the combination Macroduct®/conductivity testing is not recommended by
Jornal de Pediatria - Vol. 86, No. 2, 2010 91
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lobally, childhood obesity is an emerging public health problem.1 Elevated body mass index (BMI, kg/m2) in childhood is associated with 1) hyperlipidemia, insulin resistance, and hypertension2; and 2) adulthood obesity and cardiovascular disease (CVD).3,4 Inmany developing countries, low birth weight, underweight, and stunting are still prevalent,5,6 which might be associated with increased CVD risk in adulthood.7 The dual burden of obesity and underweight create economic and public health challenges, especially in
countries undergoing socioeconomic transition.
In this issue of Jornal de Pediatria, Silva et al.10 compared the growth patterns of Brazilian children and adolescents with the Centers for Disease Control and
Prevention (CDC)11 and the World Health Organization (WHO)12 growth charts. This cross-sectional study analyzed data involving 41,654 students (23,328 boys and 18,326 girls) aged 7-17 years. The data were collected from public and private schools located in 23 states across the ive Brazilian regions (North, Northeast, Central West, Southeast, and South) in 2004 and 2005. Height and weight were measured by trained staff using calibrated equipment. Weight, height, and BMI were compared with corresponding age- and gender-speciic CDC and WHO reference values using Student’s t test. The study showed
gender variation in height, weight, and BMI. Boys were taller at ages 7, 13-17 years; girls were taller at ages
See related article on page 115
1. PhD, Professor, Department of Pediatrics, USDA/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX, USA.
2. MD, PhD. Post-doctoral Fellow, Department of Pediatrics, USDA/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX, USA.
No conflicts of interest declared concerning the publication of this editorial.
Suggested citation: Butte NF, Nguyen TT. Is obesity an emerging problem in Brazilian children and adolescents? J Pediatr (Rio J). 2010;86(2):91-92. doi:10.2223/JPED.1998
is obesity an emerging problem
in Brazilian children and adolescents?
Nancy F. Butte,1 tuan thanh Nguyen2
Sweat testing - Mastella G
Correspondence: Gianni Mastella
E-mail: gianni.mastella@azosp.vr.it 4. Shwachman H, Dunham R, Philips WR. Electrical conductivity
of sweat: a simple diagnostic test in children. Pediatrics. 1963;32:85-8.
5. Hammond KB, Turcios NL, Gibson LE. Clinical evaluation of the macroduct sweat collection system and conductivity analyzer in the diagnosis of cystic ibrosis.J Pediatr. 1994;124:255-60. 6. Heeley ME, Woolf DA, Heeley AF. Indirect measurement of sweat
electrolyte concentrations in the laboratory diagnosis of cystic ibrosis.Arch Dis Child. 2000;82:420-4.
7. Mastella G, Di Cesare G, Borruso A, Menin L, Zanolla L. Reliability of sweat-testing by the Macroduct collection method combined with conductivity analysis in comparison with the classic Gibson and Cooke technique. Acta Paediatr. 2000;89:933-7.
8. Lezana JL, Vargas MH, Karam-Bechara J, Aldana RS, Furuya ME. Sweat conductivity and chloride titration for cystic ibrosis diagnosis in 3834 subjects. J Cyst Fibros. 2003;2:1-7.
9. Katherisan N, Gupta A, Mumford S, Cade A, Jones R. Sweat conductivity for the diagnosis of cystic ibrosis. J Cyst Fibros. 2004;3:2005.
10. Mattar AC, Gomes EN, Adde FV, Leone C, Rodrigues JC. Comparison between classic Gibson and Cooke technique and sweat conductivity test in patients with or without cystic ibrosis. J Pediatr (Rio J). 2010;86:109-14.
11. Clinical Laboratory Standard Institute. Sweat testing: sample collection and quantitative analysis: approved guideline. NCCLS Document C34-A2. Wayne, PA, National Committee for Clinical Laboratory Standards; 2000.
12. Green A, Kirk J; Guideline Development Group. Guidelines for the performance of the sweat test for the diagnosis of cystic ibrosis. Ann Clin Biochem. 2007;44:25-34.
13. Losty HC, Wheatley H, Doull I. The evaluation of a novel conductometric device for the diagnosis of cystic ibrosis. Ann Clin Biochem. 2006;43:375-81.
